This invention relates to light absorbance monitors.
In one class of light absorbance monitor, light from a gas tube discharge lamp is split into two beams. One of the beams of light is passed through an effluent to sense the absorbance of the effluent and the other is used as a reference beam.
After the sensing beam of light has passed through the effluent, both beams are converted to electrical signals. The electrical signals are compared to remove the indications relating to the reference values and common-mode noise, leaving a signal which is indicative of the absorbance or the transmittance of light. The signal is processed to obtain a visible indication of the effluent's light absorbance or light transmittance. This indication may be recorded or simply indicated on a meter.
In a prior art absorbance meter of this type, the light source is a low-pressure mercury vapor lamp. Noise is reduced from the light in this lamp prior to splitting the beam by passing it through an aperture which may or may not include a fluorescent screen.
This type of absorbance monitor has a disadvantage in that some noise remains which relates to fluctuations in common-mode light intensity. Moreover, the frequency selection of light from such sources is limited.